Specific somatically acquired chromosomal translocations as a mechanism for oncogene activation are common in hematopoietic tumors and sarcomas, but have not been demonstrated in epithelial tumors. We have established a cell line, HCC2429, from an aggressive, metastatic lung cancer arising in a young woman with a minimal smoking history that had a normal karyotype except for a single translocation between chromosomes 15q and 19p. The cell line was established and grows exceptionally well and stably maintains this translocation as its only karyotypic abnormality, suggesting a potent transforming activity. We have mapped the breakpoint of this translocation on chromosome 19 to a site approximately 40 kb upstream from the start site of Notch3, a member of Notch proto-oncogene family not previously associated with cancer. This translocation is associated with massive overexpression of Notch3, supporting the hypothesis that the t(15;19) translocation results in the deregulation of this putative cellular proto-oncogene. We have preliminary data that about 25 percent of non-small cell lung adenocarcinomas express Notch3 and no squamous or small cell tumors. FISH has identified rearrangements in the 5? region of Notch 3 in at least some of these tumors as well. We have thus identified recurring specific translocation as a novel mechanism for oncogene activation in human lung cancer as well as a novel putative oncogene not previously known to be involved in human cancer. In this proposal, we will complete the molecular characterization of the identified Translocations, determine the frequency and clinical significance of Notch3 receptor and ligand over-expression in lung cancer and normal tissues, and perform studies to characterize the transforming nature of Notch3 and its effects on downstream signaling pathways in lung cancer.